As non-volatile programmable circuit elements, floating-gate (FG) transistors have been extensively used for designing EEPROMs and flash memories, for designing analog signal processors, and for designing energy scavenging sensors. In particular, when a large number of on-chip voltage and current biases are required, as is the case for analog neural-network ICs and field-programmable analog arrays (FPAAS), floating-gates provide an ultra-compact approach for implementing field-programmable biases. Also, due to the non-volatile nature of the FG transistors, the bias values are retained across brown-outs and power-outages, making the technology also attractive for energy scavenging sensors.
The common method for programming FG transistors is by using Fowler-Nordheim (FN) tunneling or by using hot-electron injection. The procedure is illustrated in FIGS. 1A and 1B which shows the cross-sectional area of an FG p-channel MOS (pMOS) transistor and its layout. The polysilicon gate of the pMOS transistor is electrically insulated by silicon-dioxide (hence the name “floating-gate”), and any electron injected onto the gate is retained for a long period of time (8 bits precision for 8 years). FN tunneling removes the electrons from FG node by applying a high-voltage V_tun (>15V in 0.5 μm CMOS process) across a parasitic nMOS capacitor C_tun. However, the use of high-voltage also restricts the usage of FN tunneling for selective programming and therefore it is only employed to globally remove the electrons from all on-chip floating-gates.
Hot-electron injection, however, requires lower voltage (≈4.2V in 0.5 μm CMOS process) than tunneling and hence is the primary mechanism for selective programming of floating-gates. The hot-electron programming procedure, as shown in FIG. 1A, involves selection of the FG transistor (using switches) followed by applying V_sd>4.2V across the source and the drain terminals. The large electric field near the drain of the pMOS transistor creates impact-ionized hot-electrons whose energy when exceeds the gate-oxide potential barrier (≈3.2 eV) can get injected onto the floating-gate. Because the hot-electron injection in a pMOS transistor is a positive feedback process and can only be used to add electrons to the floating-gate, the process needs to be carefully controlled and periodically monitored to ensure the floating-gate voltage is programmed to a desired precision. The methods proposed in literature achieve the desired precision either by adjusting the duration for which the FG transistor is injected or by adjusting the magnitude of the injection pulses.
In this disclosure, a linear hot-electron injection technique is set forth which simplifies the programming procedure and can achieve a linear programming range as large as 4V. The procedure employs an active feedback mechanism to ensure that all the non-linear factors affecting the hot-electron injection are held constant, thus achieving a linear, stable and controllable injection rate. This is unlike the feedback methods that have been previously used for programming FG memories, where an off-chip amplifier circuit is used for indirect programming. The proposed linear injection technique results in a much simpler and more predictable programming procedure.
Additionally, the proposed linear injection technique is then used to develop a self-powered static-strain sensor.
This section provides background information related to the present disclosure which is not necessarily prior art.